I wanted to share my latest project on this forum so here is my post about it:

The schematic for the GDT Driver is a modified version of the 1.3b DRSSTC Driver. I simply removed all the components for the Interrupter and OCD path. The IGBT-Brick is a Semikron SKM400GB126D Halfbridge. The transformer has 5 primary turns and 530 secondary turns using 0.28mm copper wire.

Here is a picture of the Halfbridge:

The large ferrite core (it consists of 2 U-Cores and 2 I-Cores):

The only issue i had so far is that one TVS-Diode blew up. This happened because i am using 440V TVS-Diodes and my input voltage is 380V DC high. So if there are any transients above 60V the Diodes will blow up. I am going to put 2 Diodes in series to fix this problem. The IGBT Brick ist still fine after the explosion.

Here i have a video. The power consumption is about 6500W high in this video. I estimate the arcs to be about 70cm long in total.

The Primary has 5 turns and the secondary has around 530 turns of 0.28mm wire. I got the ferrite cores from Mouser. I am using 2 I-Cores and 2 U-Cores for the large, finished core.Here are the part numbers:B67345B0001X087 (U-Core)B67345B0002X087 (I-Core)

But remember that i had to carefully grind of some material from the I-Cores because they are not made for such an application. There was a small airgap around 100 µm wide but i managed to decrease the gap to about 15 µm using sandpaper and a glass pane. This increased my output by a little bit and the whole core is more stable now.

There are no resonant capacitors like in a ZVS, just the output of the halfbridge fed directly into the primary. Keep in mind that your IGBTs/Mosfets need a bodydiode (built in or external), to absorb the Back-EMF of the primary.

I have seen this experiment done before on 4hv and tried it unsuccessfully my self. did you have to adjust your oscillator and pulse width to maximize the arc?some times there is a thin spacer added between the mating faces of the cores maybe to prevent eddy currents. A resonant cap in parallel with the primary your primary might reduce the average power you need. Do you plan on caring your experiment further?

I got a similar project, stranded in a box for some years now, to drive some old x-ray transformers and these all uses a capacitor in series with the primary coil, if I remember correct, is not a resonant capacitor, but a DC-blocking capacitor to prevent the bridge from blowing up at core saturation?

I am rather interested in your MMC (this time Multi Mini Core) design, it does not seem like you have any clamps or anything else holding it in place, except gravity. I have a lot of brick ferrite bars and could virtually build any shape of transformer by stacking them, but I have not done any of that so far as I did not think it would work that well and know how to hold the construction together.

I think you need to adjust your oscillator to exactly 50% Duty-Cycle to make the whole thing work properly. Are you using a TL494 oscillator? I am using a small, portable function generator to provide the signal. Currently, the setup is running on 20kHz, but i can go down to 8kHz before my GDT saturates. You need to be careful with your GDT and you have to make sure, that the GDT does not saturate with lower drive frequencies. Here you can calculate how many turns you need on your GDT using the information in the datasheet. It turned out to be fairly accurate for me: http://thedatastream.4hv.org/gdt_saturation.htm

If i wanted to use a resonance circuit, i would have to make sure that I drive the LC circuit with its own resonance frequency. The ZVS is self-oscillating so the resonance frequency is always hit automatically. I am also worried about high impulse currents because of the high input voltage (380V DC in my case, a ZVS runs on lower voltage) generated in the LC circuit. Such a event could get quite destructive.

Here i have the schematic of the modified 1.3b Driver in case you are interested:

@Mads

The only problem i had so far is that my bridge is getting quite warm even after short runtimes. My switching speeds are not bad, the rise/falltimes at the bridge-output are 80ns fast. Cross conduction can't be the reason either, because the inverter pulls no current when there is no load connected while the gatedrive is running. In your opinion, how hot can a heatsink get in an hard-switched inverter? I think it is normal for the IGBT-Bricks to get hotter because of the hardswitching. I am used to the stone-cold heatsink of my DRSSTC (resonant switching), so i am a little bit worried. I have already ordered a fan to keep the heatsink temperatures down, so far i did not let the heatsink get over 40°C.

I also think that this capacitor is for DC-blocking. What is the value of the capacitor? I do not think that the capacitor is a resonant capacitor like in a DRSSTC, because such a resonance circuit in an hard-switched inverter in CW mode would be very damaging for the IGBT´s.

The ferrite core is only held together by gravity, but the PVC-pipe of the secondary guides it in place. I believe that you could glue together the cores using some epoxy glue with mixed in ferrite-dust. If you want to separate the cores again, you could carefully heat the ferrite parts with a heatgun until the epoxy softens again.

I ordered a cuple of cores to mess with. does your transformer have any sort of resonance or do you think it is acting like just turns ratio? when I look at the inital arc it seems to be about 5-10kv with a lot of current behind it?

A while ago I've build a ferrite transformer too. But I used the SLR-Topology described here: https://www.stevehv.4hv.org/ccps1.htmThis topology is acts as a current source, so its short circuit proof, soft switching, due to the waveform there is no catastrophic current rise like in the drsstc for example and the insulation of the transformer winding is much easier because of the sinusoidal current waveform. I have pushed easily 2kW through simple 20A IGBTs with no big heat. The tuning is easy and there is no failure if you set the resonant frequency wrong, it only transfers not so much power and the softswitching is gone if you set the frequency to high (not for to low frequency).

I have relaunched this project and improved some things. I now found the cause for the fast temperature rise after short run times. I did not use any gate resistors before, because the gate signal looked very good and the rise/fall times were the fastest. But now I checked the gate signal while the bridge was running and I was shocked. There were large oscillations on the gate signal which brought the IGBT back into the linear region! I now added 2.3 Ohms of gate resistance which seems to be the best value after some testing.

The high transients spikes also disappeared because of my larger snubber capacitance and slower rise/fall times.

Here you can see the halfbridge inverter:

Here is a small test video with a power consumption of around 8kVA:

Right now, I am using 5 primary turns which equates to 14µH inductance with the secondary shorted. This results in 180A of peak current flow in the primary. The current signal has a triangle waveform. The peak current is so high that the primary wires repel each other when the arc ignites I am planning to run the whole setup with more power. I could reduce the primary turn count to 4, which would lead to 280A of peak current flow.

I would also like to run the bridge on 3 phases to get 565V bus voltage. But first, I need to build myself a 3 phase variac out of 3 variacs.

I did not calculate how much I need, I just checked the output of the inverter and the DC Bus with my oscilloscope while the transformer was connected. With only 1 microfarad, there were some small transient spikes left, but with 2 microfarads they are all gone.

I have just finished building my 15kW 3 Phase Variac and I am just waiting for the large DC Bus Capacitors to arrive. I will have around 740 joules of energy stored in the new DC Bus caps at 660V.

Removing the smoothing caps is not really possible, because the transformer needs a lot of current delievered in a short period of time. There can be up to 200A flowing in the primary coil. If i hook it up directly to the mains, it could cause huge problems, because i am pulling such high currents at high frequency from it.

You are right - I should have specified "and add a bit more film capacitor decoupling for the high frequencies".

I was thinking along the lines of induction cookers etc - they get good power factor by deliberately not including a large smoothing capacitor - high frequency currents are supported by smaller film caps but the overall input voltage follows the mains supply, leading to reasonable power factor.

Removing the smoothing caps is not really possible, because the transformer needs a lot of current delievered in a short period of time. There can be up to 200A flowing in the primary coil. If i hook it up directly to the mains, it could cause huge problems, because i am pulling such high currents at high frequency from it.

You are right - I should have specified "and add a bit more film capacitor decoupling for the high frequencies".

I was thinking along the lines of induction cookers etc - they get good power factor by deliberately not including a large smoothing capacitor - high frequency currents are supported by smaller film caps but the overall input voltage follows the mains supply, leading to reasonable power factor.

That only works for quasi-resonant inverters, so it kinds of stops at 2.3kW for a single 230VAC 10A outlet or you could go bigger on phase-phase 400VAC 16/32A or higher or with a huge step-up transformer, usually not very practical solutions compared to regular DCbus+inverter. But could be interesting to see a huge quasi-resonant single switch inverter doing 10kW

You can overload your fuses temporarily, but at a risk of overheating the wiring . How much depends on wiring and fuse type, but a typical 16A slow blow fuse can take 20Amps for quite some time. It might be more or less illegal to do so, depending on country if done intentionally even if any elecitrcal motor would do so temporarily upon power on

Looks like it is a 16A CEE plug though, but I guess Phoenix knwos exactly how much his fuesbox can take

Actually I think that the power line can stand the overload longer than the electrodes...

BTW - nice work. I like your current limit circuit for the variac. But wouldnt it benefit from a larger wheel, to enable faster adjustment?